The present invention relates to the process of drying materials in a tumble chamber using hot air, but more particularly it relates to the importance of controlling the cool-down cycle at the end of the drying cycle to avoid excessive wrinkling of the materials being dried.
Conventional dryers, such as those as described in U.S. Pat. No. 4,827,627, comprise a tumble chamber in which materials to be dried are placed. The materials are rotated to effect tumbling of the materials while a burner controlled by a microprocessor controller provides a stream of hot air to the chamber. The drying temperature is maintained relatively constant throughout the drying cycle by the ability of the microprocessor controller to enable and disable the burner. Once the drying cycle time has expired, conventional dryers disable the burner and enter a free tumbling cool-down cycle. During this cool-down cycle cool air is passed through the tumble chamber for a preselected period of time, such time usually being preselected by the operator. In any event, the conventional drying system results in an uncontrolled cool-down cycle whereby the temperature gradient can vary sharply depending upon the amount and type of material being dried and the nature of the drying cycle prior to entering the cool-down cycle. An uncontrolled cool-down cycle can cool a given material much more rapidly than would be desirable to comply with the manufacturer's recommendation to obtain a substantially wrinkle free final product. For certain materials many of the steps of preparation, primarily pressing, can be eliminated if proper care is taken during the drying and cool-down cycles.
Since it is an object of any drying process to diminish the steps in preparing materials for use, a controlled cool-down cycle as disclosed in the present invention can eliminate much of the wrinkling problems associated with conventional dryers. This benefits the operator or owner by minimizing time and money spent in the preparation of materials, by avoiding the labor-intensive steps, or at least minimizing the extent of such steps of pressing or steaming materials, prior to their use. The expense associated with industrial pressing equipment can also be reduced since fewer of the materials being dried will require pressing.
In order to further improve the quality of the cool-down cycle, a pre-cool-down cycle is desirable. This cycle insures that the starting temperature of the cool-down cycle is at a preselected minimum. The microprocessor has the ability to cycle a sequence of burner on and off periods following the drying cycle to reach a predetermined temperature prior to entering the cool-down cycle. This insures the ability to control cool-down as precisely as possible by consistently starting with a designated minimum temperature. It further insures that the material contained in the tumble chamber completes the drying cycles as wrinkle-free as possible.
Preferably, the microprocessor controller contains a number of drying cycle and cool-down cycle selections, available to the operator to accommodate different materials which may be dried. Such cycles would require both time and temperature settings. The operator would choose from a number of selections, but preferably would select a drying cycle at a constant temperature in the range of 200 degrees Fahrenheit for a fixed duration of time, and a cool-down cycle having another fixed duration of time. The microprocessor controller would be preprogrammed such that the minimum cool-down cycle would have a time duration which insures that the materials reach a safe handling temperature before the operator removes them from the tumble chamber.
The problem of cool-down has been addressed in the prior art. U.S. Pat. No. 4,763,425 uses a combination of sensors, automatic controls and operator inputs to determine an upper level drying temperature, a drying time and the subsequent cool-down period. It is recognized that different cool-down durations are preferred for different materials which can avoid excessive wrinkling and damage to the specific materials being dried. U.S. Pat. No. 3,381,389 also allows for the variations of the cool-down periods by providing the operator with a manually operated switch to adjust the cool-down period. The present invention seeks to further improve the cool-down cycle by controlling the cool-down temperature gradient of the cool-down cycle.